1. Field of the Invention
The present invention relates to an image forming apparatus, a front-back magnification correction method, and a computer program product.
2. Description of the Related Art
In today's commercial printing industry, a transition is taking place from the conventional offset printing to POD using electrophotographic printing devices and the like. The POD is an abbreviation of “Print on Demand”. With this, small-lot and various kinds of variable data can be printed with ease.
When the POD is used, a front-back misregistration increases as compared with that when the offset printing is used. However, with the transition from the offset printing to the POD, an accurate front-back registration equivalent to that of the offset printing is in demand.
The known factors of the front-back misregistration are a registration error in the vertical and horizontal directions, and a skew error of an image to be printed on a sheet. When a thermal fixing device is provided in an electrophotographic printing device, an image magnification error caused by the expansion and contraction of a sheet becomes another factor of the front-back misregistration, in addition to the above-described factors.
Conventionally, there is a known technique that reduces the front-back magnification errors by calculating the expansion and contraction amount of a sheet in advance, determining the front-back image magnification, and correcting the image size. To automatically correct the front-back magnification errors, the size of the sheet needs to be measured automatically. As a technique that automatically measures the size of a sheet, there is a known technique that measures the sheet length from the time required for the leading edge of a sheet being conveyed to pass in front of a sensor, until the trailing edge of the sheet to pass in front of the sensor. There is also a known technique that measures the sheet length by counting the pulses from a rotary encoder mounted on the shaft of a sheet conveyance roller.
Japanese Patent Application Laid-open No. 2013-053004 discloses a sheet conveying device that can accurately calculate the conveying distance of a sheet with a simple configuration. With this sheet conveying device, an image is formed so as to compensate the deformation of a sheet caused by the fixing device, by correcting the image magnification according to the measured sheet size, and by changing the writing position. Hence, even if the sheet is deformed, it is possible to print an image with highly accurate front-back registration.
However, the sheet, to which the front-back magnification correction can be applied, is limited to a sheet with detectable edges, to measure the sheet length. However, for example, optical sensors have difficulties in detecting the edges of a transparent overhead projector (OHP) sheet and a black paper. Thus, when a sheet, the edges of which are difficult to detect, such as a transparent OHP sheet or a black paper is used in the image forming apparatus that detects the edges of a sheet by using an optical sensor, it is difficult to accurately measure the sheet length. Hence, an optimum front-back magnification correction value cannot be calculated, and good printing results cannot be obtained.
In this manner, in the conventional image forming apparatus, the type of the sheet used for printing is set and input by a user in advance. When the user sets a sheet, the edges of which are difficult to detect, such as a transparent OHP sheet or a black paper, the image forming apparatus performs printing without performing the front-back magnification correction.
In other words, when the sheet, the edges of which are difficult to detect, such as a transparent OHP sheet or a black paper is used, the conventional image forming apparatus performs printing at the cost of front-back registration accuracy. The type of sheets to be used for printing varies by the user. Thus, it is preferable that an accurate front-back magnification correction can be applied to various types of sheets. It is also preferable to obtain good printing results.
In view of the above, there is a need to provide an image forming apparatus, a front-back magnification correction method, and a computer program product that enable more accurate front-back magnification correction on various types of sheets, and that can obtain good printing results.